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Please give me a hint to solving this simple vector dot product proof

  1. Feb 12, 2012 #1
    1. The problem statement, all variables and given/known data

    Let 'u' and 'v' be two non zero vectors such that the prjection of 'u' along 'v' equals the projection of 'v' along 'u.' Using the formula for projection, show that 'u' and 'v' are either perpendicular or parallel.

    2. Relevant equations

    3. The attempt at a solution

    Please don't just answer it, I would like to do this one on my own. But I first need a hint because I have been trying for about 30 minutes.

    I know that the projection of 'u' along 'v' is u dot v, divided by the square of the norm of 'v'. Then this scalar is multiplied through 'v'. But that's about all I have.

    Edit: I guess I said more about how far I got. I get the following:

    Last edited: Feb 12, 2012
  2. jcsd
  3. Feb 12, 2012 #2


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    Think about what the identity $$
    \vec A\cdot \vec B = |\vec A||\vec B|\cos\theta$$implies about this problem.
  4. Feb 14, 2012 #3
    (sorry for the long time before replying)

    Thanks! That hint definitely helped me out!

    In case anyone comes by this thread seeking the same as I did, when you apply the identity that LCKurtz showed us, you get a scalar number multiplied by vector v equals vector u. Obviously, this means they are either parallel or anti-parallel.
  5. Feb 14, 2012 #4


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    Unless ##\cos\theta=0##, in which case ...
  6. Feb 14, 2012 #5
    But then wouldn't that just mean that they are parallel anyways? So no matter how you look at it, if proj u over v equals proj v over u, as long as these aren't zero vectors, wouldn't the angle HAVE to be 0 or 180?
  7. Feb 14, 2012 #6


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    What do you get for ##\vec i \cdot \vec j##?
  8. Feb 14, 2012 #7
    Oh I see.. so then given no extra information about u and v, they both can be EITHER parallel OR perpendicular? I guess I misread the question.. I thought it was asking to show that it was one OR the other, and because of this, I blindly looked for one until I found it, and then stopped before I checked the other.

    Definitely one vector dotted into another equals 0 if they are both perpendicular.

    Thanks again!
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